text: /*-------------------------------------------------------------------------
   *
   * storage.c
   *	  code to create and destroy physical storage for relations
   *
   * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
   * Portions Copyright (c) 1994, Regents of the University of California
   *
   *
   * IDENTIFICATION
   *	  src/backend/catalog/storage.c
   *
   * NOTES
   *	  Some of this code used to be in storage/smgr/smgr.c, and the
   *	  function names still reflect that.
   *
   *-------------------------------------------------------------------------
   */
  
  #include "postgres.h"
  
  #include "access/parallel.h"
  #include "access/visibilitymap.h"
  #include "access/xact.h"
  #include "access/xlog.h"
  #include "access/xloginsert.h"
  #include "access/xlogutils.h"
  #include "catalog/storage.h"
  #include "catalog/storage_xlog.h"
  #include "miscadmin.h"
  #include "storage/freespace.h"
  #include "storage/smgr.h"
  #include "utils/hsearch.h"
  #include "utils/memutils.h"
  #include "utils/rel.h"
  
  /* GUC variables */
  int			wal_skip_threshold = 2048;	/* in kilobytes */
  
  /*
   * We keep a list of all relations (represented as RelFileNode values)
   * that have been created or deleted in the current transaction.  When
   * a relation is created, we create the physical file immediately, but
   * remember it so that we can delete the file again if the current
   * transaction is aborted.  Conversely, a deletion request is NOT
   * executed immediately, but is just entered in the list.  When and if
   * the transaction commits, we can delete the physical file.
   *
   * To handle subtransactions, every entry is marked with its transaction
   * nesting level.  At subtransaction commit, we reassign the subtransaction's
   * entries to the parent nesting level.  At subtransaction abort, we can
   * immediately execute the abort-time actions for all entries of the current
   * nesting level.
   *
   * NOTE: the list is kept in TopMemoryContext to be sure it won't disappear
   * unbetimes.  It'd probably be OK to keep it in TopTransactionContext,
   * but I'm being paranoid.
   */
  
  typedef struct PendingRelDelete
  {
  	RelFileNode relnode;		/* relation that may need to be deleted */
  	BackendId	backend;		/* InvalidBackendId if not a temp rel */
  	bool		atCommit;		/* T=delete at commit; F=delete at abort */
  	int			nestLevel;		/* xact nesting level of request */
  	struct PendingRelDelete *next;	/* linked-list link */
  } PendingRelDelete;
  
  typedef struct PendingRelSync
  {
  	RelFileNode rnode;
  	bool		is_truncated;	/* Has the file experienced truncation? */
  } PendingRelSync;
  
  static PendingRelDelete *pendingDeletes = NULL; /* head of linked list */
  HTAB	   *pendingSyncHash = NULL;
  
  
  /*
   * AddPendingSync
   *		Queue an at-commit fsync.
   */
  static void
  AddPendingSync(const RelFileNode *rnode)
  {
  	PendingRelSync *pending;
  	bool		found;
  
  	/* create the hash if not yet */
  	if (!pendingSyncHash)
  	{
  		HASHCTL		ctl;
  
  		ctl.keysize = sizeof(RelFileNode);
  		ctl.entrysize = sizeof(PendingRelSync);
  		ctl.hcxt = TopTransactionContext;
  		pendingSyncHash = hash_create("pending sync hash", 16, &ctl,
  									  HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
  	}
  
  	pending = hash_search(pendingSyncHash, rnode, HASH_ENTER, &found);
  	Assert(!found);
  	pending->is_truncated = false;
  }
  
  /*
   * RelationCreateStorage
   *		Create physical storage for a relation.
   *
   * Create the underlying disk file storage for the relation. This only
   * creates the main fork; additional forks are created lazily by the
   * modules that need them.
   *
   * This function is transactional. The creation is WAL-logged, and if the
   * transaction aborts later on, the storage will be destroyed.
   */
  SMgrRelation
  RelationCreateStorage(RelFileNode rnode, char relpersistence)
  {
  	PendingRelDelete *pending;
  	SMgrRelation srel;
  	BackendId	backend;
  	bool		needs_wal;
  
  	Assert(!IsInParallelMode());	/* couldn't update pendingSyncHash */
  
  	switch (relpersistence)
  	{
  		case RELPERSISTENCE_TEMP:
  			backend = BackendIdForTempRelations();
  			needs_wal = false;
  			break;
  		case RELPERSISTENCE_UNLOGGED:
  			backend = InvalidBackendId;
  			needs_wal = false;
  			break;
  		case RELPERSISTENCE_PERMANENT:
  			backend = InvalidBackendId;
  			needs_wal = true;
  			break;
  		default:
  			elog(ERROR, "invalid relpersistence: %c", relpersistence);
  			return NULL;		/* placate compiler */
  	}
  
  	srel = smgropen(rnode, backend);
  	smgrcreate(srel, MAIN_FORKNUM, false);
  
  	if (needs_wal)
  		log_smgrcreate(&srel->smgr_rnode.node, MAIN_FORKNUM);
  
  	/* Add the relation to the list of stuff to delete at abort */
  	pending = (PendingRelDelete *)
  		MemoryContextAlloc(TopMemoryContext, sizeof(PendingRelDelete));
  	pending->relnode = rnode;
  	pending->backend = backend;
  	pending->atCommit = false;	/* delete if abort */
  	pending->nestLevel = GetCurrentTransactionNestLevel();
  	pending->next = pendingDeletes;
  	pendingDeletes = pending;
  
  	if (relpersistence == RELPERSISTENCE_PERMANENT && !XLogIsNeeded())
  	{
  		Assert(backend == InvalidBackendId);
  		AddPendingSync(&rnode);
  	}
  
  	return srel;
  }
  
  /*
   * Perform XLogInsert of an XLOG_SMGR_CREATE record to WAL.
   */
  void
  log_smgrcreate(const RelFileNode *rnode, ForkNumber forkNum)
  {
  	xl_smgr_create xlrec;
  
  	/*
  	 * Make an XLOG entry reporting the file creation.
  	 */
  	xlrec.rnode = *rnode;
  	xlrec.forkNum = forkNum;
  
  	XLogBeginInsert();
  	XLogRegisterData((char *) &xlrec, sizeof(xlrec));
  	XLogInsert(RM_SMGR_ID, XLOG_SMGR_CREATE | XLR_SPECIAL_REL_UPDATE);
  }
  
  /*
   * RelationDropStorage
   *		Schedule unlinking of physical storage at transaction commit.
   */
  void
  RelationDropStorage(Relation rel)
  {
  	PendingRelDelete *pending;
  
  	/* Add the relation to the list of stuff to delete at commit */
  	pending = (PendingRelDelete *)
  		MemoryContextAlloc(TopMemoryContext, sizeof(PendingRelDelete));
  	pending->relnode = rel->rd_node;
  	pending->backend = rel->rd_backend;
  	pending->atCommit = true;	/* delete if commit */
  	pending->nestLevel = GetCurrentTransactionNestLevel();
  	pending->next = pendingDeletes;
  	pendingDeletes = pending;
  
  	/*
  	 * NOTE: if the relation was created in this transaction, it will now be
  	 * present in the pending-delete list twice, once with atCommit true and
  	 * once with atCommit false.  Hence, it will be physically deleted at end
  	 * of xact in either case (and the other entry will be ignored by
  	 * smgrDoPendingDeletes, so no error will occur).  We could instead remove
  	 * the existing list entry and delete the physical file immediately, but
  	 * for now I'll keep the logic simple.
  	 */
  
  	RelationCloseSmgr(rel);
  }
  
  /*
   * RelationPreserveStorage
   *		Mark a relation as not to be deleted after all.
   *
   * We need this function because relation mapping changes are committed
   * separately from commit of the whole transaction, so it's still possible
   * for the transaction to abort after the mapping update is done.
   * When a new physical relation is installed in the map, it would be
   * scheduled for delete-on-abort, so we'd delete it, and be in trouble.
   * The relation mapper fixes this by telling us to not delete such relations
   * after all as part of its commit.
   *
   * We also use this to reuse an old build of an index during ALTER TABLE, this
   * time removing the delete-at-commit entry.
   *
   * No-op if the relation is not among those scheduled for deletion.
   */
  void
  RelationPreserveStorage(RelFileNode rnode, bool atCommit)
  {
  	PendingRelDelete *pending;
  	PendingRelDelete *prev;
  	PendingRelDelete *next;
  
  	prev = NULL;
  	for (pending = pendingDeletes; pending != NULL; pending = next)
  	{
  		next = pending->next;
  		if (RelFileNodeEquals(rnode, pending->relnode)
  			&& pending->atCommit == atCommit)
  		{
  			/* unlink and delete list entry */
  			if (prev)
  				prev->next = next;
  			else
  				pendingDeletes = next;
  			pfree(pending);
  			/* prev does not change */
  		}
  		else
  		{
  			/* unrelated entry, don't touch it */
  			prev = pending;
  		}
  	}
  }
  
  /*
   * RelationTruncate
   *		Physically truncate a relation to the specified number of blocks.
   *
   * This includes getting rid of any buffers for the blocks that are to be
   * dropped.
   */
  void
  RelationTruncate(Relation rel, BlockNumber nblocks)
  {
  	bool		fsm;
  	bool		vm;
  	bool		need_fsm_vacuum = false;
  	ForkNumber	forks[MAX_FORKNUM];
  	BlockNumber blocks[MAX_FORKNUM];
  	int			nforks = 0;
  
  	/* Open it at the smgr level if not already done */
  	RelationOpenSmgr(rel);
  
  	/*
  	 * Make sure smgr_targblock etc aren't pointing somewhere past new end
  	 */
  	rel->rd_smgr->smgr_targblock = InvalidBlockNumber;
  	for (int i = 0; i <= MAX_FORKNUM; ++i)
  		rel->rd_smgr->smgr_cached_nblocks[i] = InvalidBlockNumber;
  
  	/* Prepare for truncation of MAIN fork of the relation */
  	forks[nforks] = MAIN_FORKNUM;
  	blocks[nforks] = nblocks;
  	nforks++;
  
  	/* Prepare for truncation of the FSM if it exists */
  	fsm = smgrexists(rel->rd_smgr, FSM_FORKNUM);
  	if (fsm)
  	{
  		blocks[nforks] = FreeSpaceMapPrepareTruncateRel(rel, nblocks);
  		if (BlockNumberIsValid(blocks[nforks]))
  		{
  			forks[nforks] = FSM_FORKNUM;
  			nforks++;
  			need_fsm_vacuum = true;
  		}
  	}
  
  	/* Prepare for truncation of the visibility map too if it exists */
  	vm = smgrexists(rel->rd_smgr, VISIBILITYMAP_FORKNUM);
  	if (vm)
  	{
  		blocks[nforks] = visibilitymap_prepare_truncate(rel, nblocks);
  		if (BlockNumberIsValid(blocks[nforks]))
  		{
  			forks[nforks] = VISIBILITYMAP_FORKNUM;
  			nforks++;
  		}
  	}
  
  	RelationPreTruncate(rel);
  
  	/*
  	 * Make sure that a concurrent checkpoint can't complete while truncation
  	 * is in progress.
  	 *
  	 * The truncation operation might drop buffers that the checkpoint
  	 * otherwise would have flushed. If it does, then it's essential that
  	 * the files actually get truncated on disk before the checkpoint record
  	 * is written. Otherwise, if reply begins from that checkpoint, the
  	 * to-be-truncated blocks might still exist on disk but have older
  	 * contents than expected, which can cause replay to fail. It's OK for
  	 * the blocks to not exist on disk at all, but not for them to have the
  	 * wrong contents.
  	 */
  	Assert(!MyProc->delayChkptEnd);
  	MyProc->delayChkptEnd = true;
  
  	/*
  	 * We WAL-log the truncation before actually truncating, which means
  	 * trouble if the truncation fails. If we then crash, the WAL replay
  	 * likely isn't going to succeed in the truncation either, and cause a
  	 * PANIC. It's tempting to put a critical section here, but that cure
  	 * would be worse than the disease. It would turn a usually harmless
  	 * failure to truncate, that might spell trouble at WAL replay, into a
  	 * certain PANIC.
  	 */
  	if (RelationNeedsWAL(rel))
  	{
  		/*
  		 * Make an XLOG entry reporting the file truncation.
  		 */
  		XLogRecPtr	lsn;
  		xl_smgr_truncate xlrec;
  
  		xlrec.blkno = nblocks;
  		xlrec.rnode = rel->rd_node;
  		xlrec.flags = SMGR_TRUNCATE_ALL;
  
  		XLogBeginInsert();
  		XLogRegisterData((char *) &xlrec, sizeof(xlrec));
  
  		lsn = XLogInsert(RM_SMGR_ID,
  						 XLOG_SMGR_TRUNCATE | XLR_SPECIAL_REL_UPDATE);
  
  		/*
  		 * Flush, because otherwise the truncation of the main relation might
  		 * hit the disk before the WAL record, and the truncation of the FSM
  		 * or visibility map. If we crashed during that window, we'd be left
  		 * with a truncated heap, but the FSM or visibility map would still
  		 * contain entries for the non-existent heap pages.
  		 */
  		if (fsm || vm)
  			XLogFlush(lsn);
  	}
  
  	/*
  	 * This will first remove any buffers from the buffer pool that should no
  	 * longer exist after truncation is complete, and then truncate the
  	 * corresponding files on disk.
  	 */
  	smgrtruncate(rel->rd_smgr, forks, nforks, blocks);
  
  	/* We've done all the critical work, so checkpoints are OK now. */
  	MyProc->delayChkptEnd = false;
  
  	/*
  	 * Update upper-level FSM pages to account for the truncation. This is
  	 * important because the just-truncated pages were likely marked as
  	 * all-free, and would be preferentially selected.
  	 *
  	 * NB: There's no point in delaying checkpoints until this is done.
  	 * Because the FSM is not WAL-logged, we have to be prepared for the
  	 * possibility of corruption after a crash anyway.
  	 */
  	if (need_fsm_vacuum)
  		FreeSpaceMapVacuumRange(rel, nblocks, InvalidBlockNumber);
  }
  
  /*
   * RelationPreTruncate
   *		Perform AM-independent work before a physical truncation.
   *
   * If an access method's relation_nontransactional_truncate does not call
   * RelationTruncate(), it must call this before decreasing the table size.
   */
  void
  RelationPreTruncate(Relation rel)
  {
  	PendingRelSync *pending;
  
  	if (!pendingSyncHash)
  		return;
  	RelationOpenSmgr(rel);
  
  	pending = hash_search(pendingSyncHash, &(rel->rd_smgr->smgr_rnode.node),
  						  HASH_FIND, NULL);
  	if (pending)
  		pending->is_truncated = true;
  }
  
  /*
   * Copy a fork's data, block by block.
   *
   * Note that this requires that there is no dirty data in shared buffers. If
   * it's possible that there are, callers need to flush those using
   * e.g. FlushRelationBuffers(rel).
   */
  void
  RelationCopyStorage(SMgrRelation src, SMgrRelation dst,
  					ForkNumber forkNum, char relpersistence)
  {
  	PGAlignedBlock buf;
  	Page		page;
  	bool		use_wal;
  	bool		copying_initfork;
  	BlockNumber nblocks;
  	BlockNumber blkno;
  
  	page = (Page) buf.data;
  
  	/*
  	 * The init fork for an unlogged relation in many respects has to be
  	 * treated the same as normal relation, changes need to be WAL logged and
  	 * it needs to be synced to disk.
  	 */
  	copying_initfork = relpersistence == RELPERSISTENCE_UNLOGGED &&
  		forkNum == INIT_FORKNUM;
  
  	/*
  	 * We need to log the copied data in WAL iff WAL archiving/streaming is
  	 * enabled AND it's a permanent relation.  This gives the same answer as
  	 * "RelationNeedsWAL(rel) || copying_initfork", because we know the
  	 * current operation created a new relfilenode.
  	 */
  	use_wal = XLogIsNeeded() &&
  		(relpersistence == RELPERSISTENCE_PERMANENT || copying_initfork);
  
  	nblocks = smgrnblocks(src, forkNum);
  
  	for (blkno = 0; blkno < nblocks; blkno++)
  	{
  		/* If we got a cancel signal during the copy of the data, quit */
  		CHECK_FOR_INTERRUPTS();
  
  		smgrread(src, forkNum, blkno, buf.data);
  
  		if (!PageIsVerifiedExtended(page, blkno,
  									PIV_LOG_WARNING | PIV_REPORT_STAT))
  			ereport(ERROR,
  					(errcode(ERRCODE_DATA_CORRUPTED),
  					 errmsg("invalid page in block %u of relation %s",
  							blkno,
  							relpathbackend(src->smgr_rnode.node,
  										   src->smgr_rnode.backend,
  										   forkNum))));
  
  		/*
  		 * WAL-log the copied page. Unfortunately we don't know what kind of a
  		 * page this is, so we have to log the full page including any unused
  		 * space.
  		 */
  		if (use_wal)
  			log_newpage(&dst->smgr_rnode.node, forkNum, blkno, page, false);
  
  		PageSetChecksumInplace(page, blkno);
  
  		/*
  		 * Now write the page.  We say skipFsync = true because there's no
  		 * need for smgr to schedule an fsync for this write; we'll do it
  		 * ourselves below.
  		 */
  		smgrextend(dst, forkNum, blkno, buf.data, true);
  	}
  
  	/*
  	 * When we WAL-logged rel pages, we must nonetheless fsync them.  The
  	 * reason is that since we're copying outside shared buffers, a CHECKPOINT
  	 * occurring during the copy has no way to flush the previously written
  	 * data to disk (indeed it won't know the new rel even exists).  A crash
  	 * later on would replay WAL from the checkpoint, therefore it wouldn't
  	 * replay our earlier WAL entries. If we do not fsync those pages here,
  	 * they might still not be on disk when the crash occurs.
  	 */
  	if (use_wal || copying_initfork)
  		smgrimmedsync(dst, forkNum);
  }
  
  /*
   * RelFileNodeSkippingWAL
   *		Check if a BM_PERMANENT relfilenode is using WAL.
   *
   * Changes of certain relfilenodes must not write WAL; see "Skipping WAL for
   * New RelFileNode" in src/backend/access/transam/README.  Though it is known
   * from Relation efficiently, this function is intended for the code paths not
   * having access to Relation.
   */
  bool
  RelFileNodeSkippingWAL(RelFileNode rnode)
  {
  	if (!pendingSyncHash ||
  		hash_search(pendingSyncHash, &rnode, HASH_FIND, NULL) == NULL)
  		return false;
  
  	return true;
  }
  
  /*
   * EstimatePendingSyncsSpace
   *		Estimate space needed to pass syncs to parallel workers.
   */
  Size
  EstimatePendingSyncsSpace(void)
  {
  	long		entries;
  
  	entries = pendingSyncHash ? hash_get_num_entries(pendingSyncHash) : 0;
  	return mul_size(1 + entries, sizeof(RelFileNode));
  }
  
  /*
   * SerializePendingSyncs
   *		Serialize syncs for parallel workers.
   */
  void
  SerializePendingSyncs(Size maxSize, char *startAddress)
  {
  	HTAB	   *tmphash;
  	HASHCTL		ctl;
  	HASH_SEQ_STATUS scan;
  	PendingRelSync *sync;
  	PendingRelDelete *delete;
  	RelFileNode *src;
  	RelFileNode *dest = (RelFileNode *) startAddress;
  
  	if (!pendingSyncHash)
  		goto terminate;
  
  	/* Create temporary hash to collect active relfilenodes */
  	ctl.keysize = sizeof(RelFileNode);
  	ctl.entrysize = sizeof(RelFileNode);
  	ctl.hcxt = CurrentMemoryContext;
  	tmphash = hash_create("tmp relfilenodes",
  						  hash_get_num_entries(pendingSyncHash), &ctl,
  						  HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
  
  	/* collect all rnodes from pending syncs */
  	hash_seq_init(&scan, pendingSyncHash);
  	while ((sync = (PendingRelSync *) hash_seq_search(&scan)))
  		(void) hash_search(tmphash, &sync->rnode, HASH_ENTER, NULL);
  
  	/* remove deleted rnodes */
  	for (delete = pendingDeletes; delete != NULL; delete = delete->next)
  		if (delete->atCommit)
  			(void) hash_search(tmphash, (void *) &delete->relnode,
  							   HASH_REMOVE, NULL);
  
  	hash_seq_init(&scan, tmphash);
  	while ((src = (RelFileNode *) hash_seq_search(&scan)))
  		*dest++ = *src;
  
  	hash_destroy(tmphash);
  
  terminate:
  	MemSet(dest, 0, sizeof(RelFileNode));
  }
  
  /*
   * RestorePendingSyncs
   *		Restore syncs within a parallel worker.
   *
   * RelationNeedsWAL() and RelFileNodeSkippingWAL() must offer the correct
   * answer to parallel workers.  Only smgrDoPendingSyncs() reads the
   * is_truncated field, at end of transaction.  Hence, don't restore it.
   */
  void
  RestorePendingSyncs(char *startAddress)
  {
  	RelFileNode *rnode;
  
  	Assert(pendingSyncHash == NULL);
  	for (rnode = (RelFileNode *) startAddress; rnode->relNode != 0; rnode++)
  		AddPendingSync(rnode);
  }
  
  /*
   *	smgrDoPendingDeletes() -- Take care of relation deletes at end of xact.
   *
   * This also runs when aborting a subxact; we want to clean up a failed
   * subxact immediately.
   *
   * Note: It's possible that we're being asked to remove a relation that has
   * no physical storage in any fork. In particular, it's possible that we're
   * cleaning up an old temporary relation for which RemovePgTempFiles has
   * already recovered the physical storage.
   */
  void
  smgrDoPendingDeletes(bool isCommit)
  {
  	int			nestLevel = GetCurrentTransactionNestLevel();
  	PendingRelDelete *pending;
  	PendingRelDelete *prev;
  	PendingRelDelete *next;
  	int			nrels = 0,
  				maxrels = 0;
  	SMgrRelation *srels = NULL;
  
  	prev = NULL;
  	for (pending = pendingDeletes; pending != NULL; pending = next)
  	{
  		next = pending->next;
  		if (pending->nestLevel < nestLevel)
  		{
  			/* outer-level entries should not be processed yet */
  			prev = pending;
  		}
  		else
  		{
  			/* unlink list entry first, so we don't retry on failure */
  			if (prev)
  				prev->next = next;
  			else
  				pendingDeletes = next;
  			/* do deletion if called for */
  			if (pending->atCommit == isCommit)
  			{
  				SMgrRelation srel;
  
  				srel = smgropen(pending->relnode, pending->backend);
  
  				/* allocate the initial array, or extend it, if needed */
  				if (maxrels == 0)
  				{
  					maxrels = 8;
  					srels = palloc(sizeof(SMgrRelation) * maxrels);
  				}
  				else if (maxrels <= nrels)
  				{
  					maxrels *= 2;
  					srels = repalloc(srels, sizeof(SMgrRelation) * maxrels);
  				}
  
  				srels[nrels++] = srel;
  			}
  			/* must explicitly free the list entry */
  			pfree(pending);
  			/* prev does not change */
  		}
  	}
  
  	if (nrels > 0)
  	{
  		smgrdounlinkall(srels, nrels, false);
  
  		for (int i = 0; i < nrels; i++)
  			smgrclose(srels[i]);
  
  		pfree(srels);
  	}
  }
  
  /*
   *	smgrDoPendingSyncs() -- Take care of relation syncs at end of xact.
   */
  void
  smgrDoPendingSyncs(bool isCommit, bool isParallelWorker)
  {
  	PendingRelDelete *pending;
  	int			nrels = 0,
  				maxrels = 0;
  	SMgrRelation *srels = NULL;
  	HASH_SEQ_STATUS scan;
  	PendingRelSync *pendingsync;
  
  	Assert(GetCurrentTransactionNestLevel() == 1);
  
  	if (!pendingSyncHash)
  		return;					/* no relation needs sync */
  
  	/* Abort -- just throw away all pending syncs */
  	if (!isCommit)
  	{
  		pendingSyncHash = NULL;
  		return;
  	}
  
  	AssertPendingSyncs_RelationCache();
  
  	/* Parallel worker -- just throw away all pending syncs */
  	if (isParallelWorker)
  	{
  		pendingSyncHash = NULL;
  		return;
  	}
  
  	/* Skip syncing nodes that smgrDoPendingDeletes() will delete. */
  	for (pending = pendingDeletes; pending != NULL; pending = pending->next)
  		if (pending->atCommit)
  			(void) hash_search(pendingSyncHash, (void *) &pending->relnode,
  							   HASH_REMOVE, NULL);
  
  	hash_seq_init(&scan, pendingSyncHash);
  	while ((pendingsync = (PendingRelSync *) hash_seq_search(&scan)))
  	{
  		ForkNumber	fork;
  		BlockNumber nblocks[MAX_FORKNUM + 1];
  		BlockNumber total_blocks = 0;
  		SMgrRelation srel;
  
  		srel = smgropen(pendingsync->rnode, InvalidBackendId);
  
  		/*
  		 * We emit newpage WAL records for smaller relations.
  		 *
  		 * Small WAL records have a chance to be emitted along with other
  		 * backends' WAL records.  We emit WAL records instead of syncing for
  		 * files that are smaller than a certain threshold, expecting faster
  		 * commit.  The threshold is defined by the GUC wal_skip_threshold.
  		 */
  		if (!pendingsync->is_truncated)
  		{
  			for (fork = 0; fork <= MAX_FORKNUM; fork++)
  			{
  				if (smgrexists(srel, fork))
  				{
  					BlockNumber n = smgrnblocks(srel, fork);
  
  					/* we shouldn't come here for unlogged relations */
  					Assert(fork != INIT_FORKNUM);
  					nblocks[fork] = n;
  					total_blocks += n;
  				}
  				else
  					nblocks[fork] = InvalidBlockNumber;
  			}
  		}
  
  		/*
  		 * Sync file or emit WAL records for its contents.
  		 *
  		 * Although we emit WAL record if the file is small enough, do file
  		 * sync regardless of the size if the file has experienced a
  		 * truncation. It is because the file would be followed by trailing
  		 * garbage blocks after a crash recovery if, while a past longer file
  		 * had been flushed out, we omitted syncing-out of the file and
  		 * emitted WAL instead.  You might think that we could choose WAL if
  		 * the current main fork is longer than ever, but there's a case where
  		 * main fork is longer than ever but FSM fork gets shorter.
  		 */
  		if (pendingsync->is_truncated ||
  			total_blocks * BLCKSZ / 1024 >= wal_skip_threshold)
  		{
  			/* allocate the initial array, or extend it, if needed */
  			if (maxrels == 0)
  			{
  				maxrels = 8;
  				srels = palloc(sizeof(SMgrRelation) * maxrels);
  			}
  			else if (maxrels <= nrels)
  			{
  				maxrels *= 2;
  				srels = repalloc(srels, sizeof(SMgrRelation) * maxrels);
  			}
  
  			srels[nrels++] = srel;
  		}
  		else
  		{
  			/* Emit WAL records for all blocks.  The file is small enough. */
  			for (fork = 0; fork <= MAX_FORKNUM; fork++)
  			{
  				int			n = nblocks[fork];
  				Relation	rel;
  
  				if (!BlockNumberIsValid(n))
  					continue;
  
  				/*
  				 * Emit WAL for the whole file.  Unfortunately we don't know
  				 * what kind of a page this is, so we have to log the full
  				 * page including any unused space.  ReadBufferExtended()
  				 * counts some pgstat events; unfortunately, we discard them.
  				 */
  				rel = CreateFakeRelcacheEntry(srel->smgr_rnode.node);
  				log_newpage_range(rel, fork, 0, n, false);
  				FreeFakeRelcacheEntry(rel);
  			}
  		}
  	}
  
  	pendingSyncHash = NULL;
  
  	if (nrels > 0)
  	{
  		smgrdosyncall(srels, nrels);
  		pfree(srels);
  	}
  }
  
  /*
   * smgrGetPendingDeletes() -- Get a list of non-temp relations to be deleted.
   *
   * The return value is the number of relations scheduled for termination.
   * *ptr is set to point to a freshly-palloc'd array of RelFileNodes.
   * If there are no relations to be deleted, *ptr is set to NULL.
   *
   * Only non-temporary relations are included in the returned list.  This is OK
   * because the list is used only in contexts where temporary relations don't
   * matter: we're either writing to the two-phase state file (and transactions
   * that have touched temp tables can't be prepared) or we're writing to xlog
   * (and all temporary files will be zapped if we restart anyway, so no need
   * for redo to do it also).
   *
   * Note that the list does not include anything scheduled for termination
   * by upper-level transactions.
   */
  int
  smgrGetPendingDeletes(bool forCommit, RelFileNode **ptr)
  {
  	int			nestLevel = GetCurrentTransactionNestLevel();
  	int			nrels;
  	RelFileNode *rptr;
  	PendingRelDelete *pending;
  
  	nrels = 0;
  	for (pending = pendingDeletes; pending != NULL; pending = pending->next)
  	{
  		if (pending->nestLevel >= nestLevel && pending->atCommit == forCommit
  			&& pending->backend == InvalidBackendId)
  			nrels++;
  	}
  	if (nrels == 0)
  	{
  		*ptr = NULL;
  		return 0;
  	}
  	rptr = (RelFileNode *) palloc(nrels * sizeof(RelFileNode));
  	*ptr = rptr;
  	for (pending = pendingDeletes; pending != NULL; pending = pending->next)
  	{
  		if (pending->nestLevel >= nestLevel && pending->atCommit == forCommit
  			&& pending->backend == InvalidBackendId)
  		{
  			*rptr = pending->relnode;
  			rptr++;
  		}
  	}
  	return nrels;
  }
  
  /*
   *	PostPrepare_smgr -- Clean up after a successful PREPARE
   *
   * What we have to do here is throw away the in-memory state about pending
   * relation deletes.  It's all been recorded in the 2PC state file and
   * it's no longer smgr's job to worry about it.
   */
  void
  PostPrepare_smgr(void)
  {
  	PendingRelDelete *pending;
  	PendingRelDelete *next;
  
  	for (pending = pendingDeletes; pending != NULL; pending = next)
  	{
  		next = pending->next;
  		pendingDeletes = next;
  		/* must explicitly free the list entry */
  		pfree(pending);
  	}
  }
  
  
  /*
   * AtSubCommit_smgr() --- Take care of subtransaction commit.
   *
   * Reassign all items in the pending-deletes list to the parent transaction.
   */
  void
  AtSubCommit_smgr(void)
  {
  	int			nestLevel = GetCurrentTransactionNestLevel();
  	PendingRelDelete *pending;
  
  	for (pending = pendingDeletes; pending != NULL; pending = pending->next)
  	{
  		if (pending->nestLevel >= nestLevel)
  			pending->nestLevel = nestLevel - 1;
  	}
  }
  
  /*
   * AtSubAbort_smgr() --- Take care of subtransaction abort.
   *
   * Delete created relations and forget about deleted relations.
   * We can execute these operations immediately because we know this
   * subtransaction will not commit.
   */
  void
  AtSubAbort_smgr(void)
  {
  	smgrDoPendingDeletes(false);
  }
  
  void
  smgr_redo(XLogReaderState *record)
  {
  	XLogRecPtr	lsn = record->EndRecPtr;
  	uint8		info = XLogRecGetInfo(record) & ~XLR_INFO_MASK;
  
  	/* Backup blocks are not used in smgr records */
  	Assert(!XLogRecHasAnyBlockRefs(record));
  
  	if (info == XLOG_SMGR_CREATE)
  	{
  		xl_smgr_create *xlrec = (xl_smgr_create *) XLogRecGetData(record);
  		SMgrRelation reln;
  
  		reln = smgropen(xlrec->rnode, InvalidBackendId);
  		smgrcreate(reln, xlrec->forkNum, true);
  	}
  	else if (info == XLOG_SMGR_TRUNCATE)
  	{
  		xl_smgr_truncate *xlrec = (xl_smgr_truncate *) XLogRecGetData(record);
  		SMgrRelation reln;
  		Relation	rel;
  		ForkNumber	forks[MAX_FORKNUM];
  		BlockNumber blocks[MAX_FORKNUM];
  		int			nforks = 0;
  		bool		need_fsm_vacuum = false;
  
  		reln = smgropen(xlrec->rnode, InvalidBackendId);
  
  		/*
  		 * Forcibly create relation if it doesn't exist (which suggests that
  		 * it was dropped somewhere later in the WAL sequence).  As in
  		 * XLogReadBufferForRedo, we prefer to recreate the rel and replay the
  		 * log as best we can until the drop is seen.
  		 */
  		smgrcreate(reln, MAIN_FORKNUM, true);
  
  		/*
  		 * Before we perform the truncation, update minimum recovery point to
  		 * cover this WAL record. Once the relation is truncated, there's no
  		 * going back. The buffer manager enforces the WAL-first rule for
  		 * normal updates to relation files, so that the minimum recovery
  		 * point is always updated before the corresponding change in the data
  		 * file is flushed to disk. We have to do the same manually here.
  		 *
  		 * Doing this before the truncation means that if the truncation fails
  		 * for some reason, you cannot start up the system even after restart,
  		 * until you fix the underlying situation so that the truncation will
  		 * succeed. Alternatively, we could update the minimum recovery point
  		 * after truncation, but that would leave a small window where the
  		 * WAL-first rule could be violated.
  		 */
  		XLogFlush(lsn);
  
  		/* Prepare for truncation of MAIN fork */
  		if ((xlrec->flags & SMGR_TRUNCATE_HEAP) != 0)
  		{
  			forks[nforks] = MAIN_FORKNUM;
  			blocks[nforks] = xlrec->blkno;
  			nforks++;
  
  			/* Also tell xlogutils.c about it */
  			XLogTruncateRelation(xlrec->rnode, MAIN_FORKNUM, xlrec->blkno);
  		}
  
  		/* Prepare for truncation of FSM and VM too */
  		rel = CreateFakeRelcacheEntry(xlrec->rnode);
  
  		if ((xlrec->flags & SMGR_TRUNCATE_FSM) != 0 &&
  			smgrexists(reln, FSM_FORKNUM))
  		{
  			blocks[nforks] = FreeSpaceMapPrepareTruncateRel(rel, xlrec->blkno);
  			if (BlockNumberIsValid(blocks[nforks]))
  			{
  				forks[nforks] = FSM_FORKNUM;
  				nforks++;
  				need_fsm_vacuum = true;
  			}
  		}
  		if ((xlrec->flags & SMGR_TRUNCATE_VM) != 0 &&
  			smgrexists(reln, VISIBILITYMAP_FORKNUM))
  		{
  			blocks[nforks] = visibilitymap_prepare_truncate(rel, xlrec->blkno);
  			if (BlockNumberIsValid(blocks[nforks]))
  			{
  				forks[nforks] = VISIBILITYMAP_FORKNUM;
  				nforks++;
  			}
  		}
  
  		/* Do the real work to truncate relation forks */
  		if (nforks > 0)
  			smgrtruncate(reln, forks, nforks, blocks);
  
  		/*
  		 * Update upper-level FSM pages to account for the truncation. This is
  		 * important because the just-truncated pages were likely marked as
  		 * all-free, and would be preferentially selected.
  		 */
  		if (need_fsm_vacuum)
  			FreeSpaceMapVacuumRange(rel, xlrec->blkno,
  									InvalidBlockNumber);
  
  		FreeFakeRelcacheEntry(rel);
  	}
  	else
  		elog(PANIC, "smgr_redo: unknown op code %u", info);
  }
  